I do not think that tapping would dislodge some sort of blockage. I believe it is just a sticking expansion valve. Your results with the low side in a vacuum definately show a stuck expansion valve. Fwiw, I lost all of my R12 when a brand new expansion valve stuck shut on my 300TD. Exact same symptoms. A flush would not be a bad idea, along with a new drier and replacing all o rings and fresh a/c oil.

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1983 300TD Wagon